Selected reaction monitoring, specificity

Selected Reaction Monitoring (SRM) Data acquired from specific product ions corresponding to m/z selected precursor ions recorded via two or more stages of mass spectrometry. Selected reaction monitoring can be preformed as tandem mass spectrometry in time or tandem mass spectrometry in space. The term multiple reaction monitoring is deprecated [1],... 

The standard addition procedure is another method for recognising and overcoming potential matrix effects in quantification. Both alternatives, FIA—MS or FIA—MS—MS, can be performed using this procedure. Despite the increased expenditure because of a multiplication in analyses, the FIA approach combined with standard addition remains the faster technique even with the application of specific analytical MS—MS techniques such as product-, parent- or neutral loss scans applying selected reaction monitoring (SRM). The greatest drawback of this technique is that the compounds to be quantified must... 

Selected Reaction Monitoring (MS/MS) Selected reaction monitoring (SRM) is the process by which the first mass analysis selects a specific m/z (the precursor ion) to be fragmented in the collision cell and the second mass analysis selects and detects a specific product ion. Most commonly used in the quantitative analysis of well-characterized, targeted species for which optimized precursor-product pairs can be established. In SRM-based LC-MS assays no qualitative information can be obtained. However, SRM can be used to trigger product ion, neutral loss, or precursor ion scans. 

The HPLC/MS/MS assays of other CYP enzymes are very similar in principle and use the identical instrumentation but employ different internal standards. As a consequence of the high degree of specificity of MS/MS selected reaction monitoring, batteries of CYP assays can be robotically programmed for high throughput with little additional manpower. 

FIGURE 12.14 Selected reaction monitoring reflects the intensity of the transitions m/z 256 to 139 and 262 to 139. Note that the peak profiles are free from interference, indicating the specificity and selectivity of the measurement. The internal standard signal is 153 times the intensity of hydroxybupropion. Data provided by R.L. Walsky and R.S. Obach, Pfizer, New York, NY. 

Selected reaction monitoring in LC/MS/MS allowed glucuronic- and sulfate-selective detection as well as specific detection of xenobiotics and their elaborated conjugates. 

Figure 7 Scan modes for a tandem-in-space instrument, the triple quadruple (QqQ). (a) Full scan all source ions are passed through to Q3 while Q1 and q (collision cell) are set to the RF-only mode, (b) Production scan Qi is set to pass a selected ion (precursor ion). This is fragmented in the collision cell and products are analyzed by scanning Q3. (c) Precursor scan Q1 scans all the source ions into the collision cell for collision-induced dissociation (CID). Q3 is set to pass a selected product ion. A signal recorded at Q3 is correlated with the corresponding precursor ion passing through Q-i. (d) Neutral loss scan Q-i is set to scan ions into the collision cell for CID. The Q3 scan is offset by a specified mass, equal to the mass of the neutral, relative to Qi. (e) Selected reaction monitoring (SRM) an ion selected in Q1 is fragmented and a specific fragment is then recorded after selection by Q3. SRM is commonly used in quantitative work to improve assay selectivity and sensitivity.

The quantification of the metabolites of CYPs 2D6, 3A4, and 2C19 reactions is carried out using a SCIEX API 3000 mass spectrometer, running in the positive ion, selected reaction monitoring (SRM) mode. The HPLC gradient system used consists of two solvent mixtures. Solvent mixture A (SMA) consists of 94.9% H20, 5% MeOH, and 0.1% formic acid and solvent mixture B consists of 94.9% MeOH, 5% H20, and 0.1% formic acid. The analytical column used is a Develosil Combi-RP5, 5 pm, 3.5x20mm (Phenomenex, Inc., Torrance, CA), with a mobile phase flow rate of 1.1 mL/min. A short run time (1 min) HPLC method is used because of the specificity of the mass spectrometer and the lack of matrix effects (this was thoroughly... 

SRM (Selected Reaction Monitoring) In MS/MS, a specific dissociation of a parent to daughter ion is monitored. lUPAC recommendations for terms for use in mass spectrometry can be found in (Todd 1991). 

In both tandem-in-space and tandem-in-time instruments, the most common experiment is for the first analyzer to select specific ions from the total ion beam arriving from the ion source. Next, the selected ions undergo collision-induced dissociation (CID) in a pressurized cell followed by the analysis of the product ions in the second analyzer. In tandem-in-time the same analyzer is used for both scans, but at different times. The resulting product ion spectra (or precursor and neutral loss spectra in other forms of MS/MS analysis) provide vital structural information for the identification of small molecules (such as drug metabolites) as well as complex biomolecules. Selected reaction monitoring (SRM), another mode of MS/MS operation, provides highly specific and sensitive quantification of target analytes. 

The intensity of the current produced by analyte ions is relevant in quantification. Limits of detection are improved when fragmentation is reduced or eliminated and the ion current, attributable to the analyte, is present as a single species. For instance, using Cl often improves both detection and quantification limits when compared to El, although the controlled fragmentation used in selected reaction monitoring can also improve detection limits. Fragmentation as it applies to specific quantification techniques for small molecules is discussed in connection with the quadrupole family of instruments (Sections 3.3.3.1 and 3.3.5). Quantification for biopolymers, particularly proteins, is presented in Section 3.5.1.9. 

A specific voltage, selected to provide a sii e intense fragment, is used in selected reaction monitoring. 

Because drug testing for controlled substances bears heavily on a person s reputation, the method to detect controlled substance must be accurate and must follow international guidelines, which advocate a method that must include chromatographic separation to isolate the analyte from other matrix components and mass spectrometric detection with either SIM of three compound-specific ions on a single mass analyzer or with selected-reaction monitoring (SRM) of two specified precursor-product reactions. GC/MS and LC/MS methods that follow these guidelines are used routinely to detect and quantify cannabinoids, narcotics, cocaine, amphetamines, and other substances of abuse [57]. 

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